Conventionally, a scroll-type compressor as an example of a compressor to compress a gas refrigerant in a refrigerating cycle has been used. The scroll-type compressor includes a fixed scroll and an orbiting scroll, which have involute wraps engaged with each other, in its casing. The fixed scroll is fixed to the casing and the orbiting scroll is coupled to an eccentric portion of a drive shaft (crank shaft). In the scroll-type compressor, the orbiting scroll just revolves orbitally to the fixed scroll without rotating on its own axis, thereby contracting a compression chamber formed between the wraps of both scrolls to compress a gas such as the refrigerant therein.
A certain compressor of this scroll-type compressor adopts a structure in which an orbiting scroll (OS) is pressed against a fixed scroll (FS) in the axis direction, as shown in FIG. 10. A purpose of this structure is to avoid a state occurring, where a refrigerant leaks and thereby an efficiency is reduced, due to inclination (overturn) of the orbiting scroll (OS) caused by so-called overturning moment that is produced by an axis-direction gas load Fz acting on the orbiting scroll (OS) by gas compression and a radius-direction load Fx which is a resultant force of gas force and centrifugal force.
It is known that the above-described axis-direction gas load Fz and radius-direction load Fx show their highest magnitudes almost at the same time, as shown in FIG. 11. Specifically, at a crank angle (revolution angle of the orbiting scroll (OS)) where an internal pressure of the compression chamber reaches about its highest pressure, these loads Fz, Fx become the highest and also the overturning moment M becomes the highest.
Problems to be Solved
Herein, it would be necessary that the magnitude of the above pressing force should be set based on the highest value of the overturning moment in order to prevent the orbiting scroll (OS) from overturning certainly during the operation of the compressor. However, just setting the pressing force at a certain magnitude that could prevent the orbiting scroll (OS) from overturning at the time the overturning moment shows its highest value would cause an overproduction of pressing force at a crank angle where the overturning moment is lower than its highest value, resulting in a lower efficiency due to some machine loss.
Meanwhile, another compressor of this scroll-type compressor adopts a structure in which refrigerating machine oil with high pressure is plied to a sliding face constituted of the fixed scroll (FS) and the orbiting scroll (OS), thereby the orbiting scroll (OS) is pressed back with a force Fo against the above pressing force. For example, Japanese Patent Laid-Open Publication No. 2001-214872 shows an adjusting structure to adjust the pressing force according to fluctuation of a compression ratio (or a pressure differential between high-level pressure and low-level pressure) based on operating conditions of the apparatus. However, this prior art compressor does not either perform adjusting a pressing-back force according to fluctuation of the above axis-direction gas load or the overturning moment during the orbital revolution of the orbiting scroll (OS). Namely, in the above compressor, the pressing-back force is generated or halted according merely to the magnitude of the compression ratio (or the high-low pressure differential), and the pressing-back force has constant strength regardless of its crank angle when it is generated. Accordingly, this compressor cannot cope with the fluctuation of the overturning moment and the like during the revolution of the orbiting scroll (OS) and thus it cannot make the orbital movement of the orbiting scroll (OS) stable sufficiently.
The present invention has been devised in view of these problems, and an object of the present invention is to make the revolution movement of the orbiting scroll stable by changing the pressing force of the orbiting scroll against the fixed scroll according to fluctuation of the axis-direction gas load or the overturning moment during the revolution of the orbiting scroll, thereby improving compression efficiency of the scroll-type compressor.